Read SDN, read up on current issues (primarily on AVMA), talked to people who interviewed there (and at other schools) in previous years
What was the most difficult question?
How do you think the conditions that lab animals are kept/housed in are? (Follow up to How do you feel about the use of animals in teaching and research?) (Only difficult because I have no experience with lab animals, so I had to admit that I do not know - which was fine!)